OPTICAL FABRICATION: Thin films provide wide-angle correction for waveplate components
Over the last several years, complex optical systems have become consumer items, significantly changing the dynamics of price to performance. Examples of this can be found in medical instruments, high-definition television (HDTV), and digital imaging for cameras and mobile phones.
Many of these new applications rely on sophisticated control of the polarization state of light. In the past, systems requiring polarization-controlling components were designed around materials that were readily available in crystals, stretched films, or tilted coatings. However, recent manufacturing advances have created technologies that can be tailored to the system need rather than requiring the system to adapt to the polarization component. The goal is to create a capability for polarization components that is analogous to the ability of thin-film filters to meet virtually any spectral performance.
Traditional retarders (or waveplates) are made from stretched-polymer foil laminated to a glass substrate, or from crystalline materials such as quartz and mica. These common birefringent materials exhibit a change in A-plate (or in-plane) retardance with angle of incidence that is a characteristic of the material. To minimize contribution from this characteristic in an optical system, the common practice for systems engineers is to position the retarder in collimated space.
An alternate approach is to introduce a C-plate (or out-of-plane) retardance to compensate. The result is a flat retardance response as a function of angle of incidence. Using its new Ucp-1 coating chamber, JDSU has developed thin-film structures that can provide such retardance compensation-even out to relatively large angles of incidence.1
Azimuthal and angular dependence
The most common type of retarder for polarization control has an optic axis that lies in the plane of the component. These A-plate retarders are typically optimized for collimated light at normal incidence to the plane of the retarder, and the extraordinary axis of the retarder material (and thus the uniaxial index ellipsoid) is perpendicular to the incoming beam.
It is well known that quartz and other uniaxially birefringent materials exhibit a change in retardance with azimuthal angle (plane of incidence relative to optic axis) and polar angle of incidence.2 This is also important for a noncollimated beam of light as retardance will change as a function of the illumination-cone ray angle, quantified as an f/number (beams with smaller f/>numbers will show greater variation across the cone). Extreme rays tend to be less or more retarded than the rays at normal incidence, depending upon azimuthal angle.
For a retarder of approximately 0.44 waves (a 277 nm retardance at λ = 633 nm; see Fig. 1), only input light with its plane of incidence aligned at 45° relative to the fast axis of the component will have its retardance remain (almost) constant with a change in angle of incidence. In the extreme cases of input light with its plane of incidence aligned at 90° to that of the filter fast axis, a change in retardance of approximately ±3% relative to that at normal incidence will be observed as the beam scans over ±20° (retarders are typically toleranced to within 1% of their nominal on-axis retardance). Thus, as the angle of incidence increases, the transmitted light will be less or more elliptically polarized than the normal incidence case, resulting in an undesired increase in crosstalk between orthogonal polarization states or a reduction in resolved intensity of the required polarization state of light.
The azimuthal and angular dependence of retardance can constrain the systems engineer to a limited scan-angle orientation or can force the optical engineer into using auxiliary optical components to collimate and refocus light, resulting in higher complexity, cost, and increased use of space to place the retarder in collimated space.
Compensation for retardance variation
It is possible to introduce out-of-plane or C-plate retardance where the axis of the extraordinary refractive index (ne) is perpendicular to the plane of the retarder. The C-plate retardance can be positive or negative in sign depending on whether ne is greater than or less than the ordinary index (no), respectively. Both conditions can be achieved by use of a thin-film high/low index stack. This out-of-plane retardance can then offset that of the change in A-plate retardance with angle, resulting in a single component whose retardance is almost invariant with angle of incidence. Additionally, broadband antireflection (AR) coatings can be incorporated into the coating layers.
In a negative C-plate design, the use of effective media theory (EMT) predicts the introduction of birefringence when large numbers (tens or even hundreds) of alternating high/low index thin-film layers with thicknesses at a fraction of the operating wavelength are utilized in a dielectric stack.3, 4 For a positive C-plate dielectric design, fewer layers are used and the layer thickness is not necessarily a small fraction of the operating wavelength.
When AR coatings are integrated into the birefringent stack, the former design is a form-birefringent AR (FBAR) whereas the latter is a positive-C AR (PCAR). The C-plate retardance of the FBAR and PCAR stacks can be designed to be anywhere from 50 to 1000 nm (positive or negative) for custom off-axis compensation requirements. The all-inorganic nature of these coatings means that they have high durability and reliability, withstanding high light-flux densities (above 40 Mlux) and high temperatures (above 120°C) for more than 10,000 hours.
Hybrid-liquid-crystal technology
Conventional foil-type and quartz A‑plate retarders have their own advantages and disadvantages.
Quartz retarders can be expensive-up to five times the price of equivalent stretched-polymer retarders-and available sizes are limited by crystal growth capabilities (typically 70 mm diameters, although recent developments in synthetic materials have pushed this out potentially to 150 mm; see www.laserfocusworld.com/articles/266369). True zero-order quartz components are extremely thin (a quarter-wave plate at 633 nm is only about 15 μm thick) and must be mounted or laminated on glass for protection. However, the material and its manufacturing and capabilities are established and well understood.
Stretched-polymer retarders are known to be less durable under conditions of high flux and/or temperature and it can be difficult to obtain custom retardance values if a specific material with the required index properties is not readily available. Laminated sheet can, however, be produced in large quantities and can be very cost-effective in some high-volume applications.
The alternative technology developed by JDSU uses a hybrid-liquid-crystal (HyLC) polymer material applied by a spin-coating process. The liquid crystal is aligned by an ultraviolet, noncontact photo-alignment process that offers many advantages over conventional “rubbing” alignment processes, including freedom from electrostatic damage and mechanical damage such as scratches. It is also possible to generate a patterned orientation structure using a mask-a feature unique to HyLC technology. The HyLC process has been standardized on 200 mm wafer substrates and is well suited to the Ucp-1 coating-chamber technology that can coat large numbers of layers and allows for high-throughput of mixed designs, making small runs of custom designs possible at reasonable cost.
The process offers accurate retardance-value targeting (within ±3%) and excellent uniformity of retardance value and orientation angle. There is negligible thermal and mechanical stress-induced birefringence and the resulting retarders have shown high reliability under high light flux (mean time to failure better than 10,000 hours at over 20 Mlux; λ above 410 nm) and elevated temperature (up to 100°C) conditions.5 The customizable process can generate components with high and low retardance (greater than 800 nm or less than 10 nm). High transmitted-wavefront quality is also attainable with a single substrate process (λ/4 standard, λ/10 precision or better). Components can be produced in sizes up to 200 mm in diameter.
Combined A- and C-plate applications
Independent customization of A- and C-plate retardance offers the opportunity to develop tailored polarization-control component solutions. In ophthalmic instrument applications, the off-axis retardance variations of the HyLC A‑plate retarder can be compensated with a PCAR dielectric stack design. These custom components can be designed such that retardance remains uniform over the full azimuthal angle range of 360° and angles of incidence up to 20° (see Fig. 2). This would enable, for example, the component to be placed in a laser scanning system without the need for telecentric lens scanning.
Other dielectric PCAR designs can be used to compensate for A-plate roll-off to larger polar angles. Moreover, the PCAR design and fabrication process can be applied to other retarder technologies such as quartz and foil retarders.
In some liquid-crystal-on-silicon (LCOS) projection applications, system contrast can be dramatically enhanced (up to a factor of 10 improvement) using a trim (low magnitude) retarder. The JDSU birefringent contrast enhancer (BCE) product offers the low retardance (less than 10 nm) necessary to correct birefringence introduced by the LCOS panel pretilt while simultaneously correcting off-axis retardance for an f/2.4 cone of illumination with an FBAR stack design.6
Such tailored design capabilities free the design engineer from the collimation-space paradigm of retarder use.
REFERENCES
1. S. Sullivan, M. Tilsch, and F. Van Milligen, Photonics Spectra 39(11)86 (November 2005).
2. P. Yeh and C. Gu, Optics of Liquid Crystal Displays, John Wiley & Sons, New York, NY (1999).
3. M. Born and E. Wolf, Principles of Optical Fabrication, 7th Ed., p. 837, Cambridge University Press, Cambridge, United Kingdom (1999).
4. K.D. Hendrix et al., J. Vacuum Science Technology A 24(4) 1546 (2006).
5. D.M. Shemo et al., SID 2006 Digest, 1038 (2006).
6. K.L. Tan et al., SID 2005 Digest, 1810 (2005).
KIM TAN is a research engineer, KAREN HENDRIX is a senior filter design engineer, and PAUL McKENZIE is engineering development manager in the Custom Optics Product Group at JDSU, 2789 Northpoint Parkway, Santa Rosa, CA 95407; e-mail: [email protected]; www.jdsu.com/customoptics.